In recent years, automotive exhaust gas regulation has been strengthened, a lean-burn engine, a direct jet engine and the like have spread, and accordingly a nitrogen oxide trap catalyst (hereinafter referred to as “NOx trap catalyst”) has been put to practical use, which is capable of effectively purifying nitrogen oxide (NOx) in an exhaust gas in a lean atmosphere. The NOx trap catalyst contains effective components such as alkali metals (potassium (K), sodium (Na), lithium (Li), cesium (Cs), etc.), alkali earth metals (barium (Ba), calcium (Ca), etc.), and rare earths (lanthanum (La), yttrium (Y), etc.). Especially, barium has been broadly used from the beginning of the practical use of the NOx trap catalyst. In recent years, addition of potassium has been attempted which is superior in a nitrogen oxide trap ability (hereinafter referred to as “NOx trap ability”).
This NOx trap catalyst is usually used in the form of a catalyst carried on a catalyst carrier formed of an oxide ceramic such as cordierite.
However, the catalyst carrier formed of the oxide ceramic easily degrades by corrosion of the alkali metal or alkali earth metal (hereinafter referred to as “the alkali metal or the like”) which is activated at a high temperature by the exhaust gas, especially lithium, sodium, potassium, calcium or the like, and there has been a problem that cracks are generated in the catalyst carrier, when the degradation progresses. Since the alkali metal or the like reacts with the catalyst carrier and is consumed, there has also been a problem that a catalyst performance drops with time.
To solve the problems, a method has been proposed in which the surface of the catalyst carrier is coated with a certain coating layer, and the NOx trap catalyst is carried on the coating layer (e.g., Japanese Patent Application Laid-Open Nos. 10-137590, 2002-59009, etc.). According to these methods, diffusion of the alkali metal or the like to the catalyst carrier, and further reaction of the alkali metal or the like with the catalyst carrier are suppressed by the coating layer, it is possible to avoid the above-described problems.
Additionally, in recent years, a nonoxide ceramic such as silicon carbide has been noted as a material constituting the catalyst carrier. The nonoxide ceramic is superior in heat resistance or chemical durability, and does not easily react with the alkali metal or the like which is the NOx trap catalyst even at the high temperature by the exhaust gas. Therefore, unlike the oxide ceramic, the problems do not occur that the cracks are generated in the catalyst carrier and the catalyst activity drops.
However, in actual, when the alkali metal or the like is carried by the catalyst carrier formed of the nonoxide ceramic, any crack is not generated in the catalyst carrier, but there has been a problem that the catalyst activity drops more then expected with use time.